1. Field of the Invention
The present invention relates to materials-recovery-type recycling of bonded magnets which are excellent in dimensional accuracy and workability, therefore, widely used as an industrial material.
2. Related Art of the Invention
Hard magnetic material of any type is generally hard and brittle; moreover, its manufacturing process usually includes casting, sintering, heat treatment, and grinding finishing as a last step so as to produce a prescribed dimension. As a result, the processing cost becomes high for applications where close dimensional tolerance is required, which leads to a remarkable increase in the cost of product. In addition, with this material, intricate shapes and thin products are hard to make.
The hard magnetic material having been improved in the above disadvantages in processability and workability and having come into use is a bonded magnet. The bonded magnet is a magnet produced by mixing hard magnetic material powder with a binder, such as plastic and rubber, and subjecting the mixture to molding forming, and it is characterized in that it has a high dimensional accuracy, it is workable into an intricate shape and into a thin product, it is free from fear of cracking and chipping, and that it has a light weight.
The magnetic properties of the bonded magnets are inferior to those of cast magnets and sintered magnets since they contain 2 to 15 wt % binder of non-magnetic material (25 to 50% by volume); however, after powerful powders of magnetic materials such as rare earth permanent magnet were obtained, their properties have been rapidly improved and they have wider applications.
The particle size of the magnetic powder of, for example, rare earth permanent magnet is made uniform, the uniform powder is mixed with a binder, and the mixture is subjected to molding forming by the compression molding, injection molding, etc. In cases where the binder is a thermosetting resin, the molded form is subjected to thermosetting subsequently. In the bonded magnets, the density and the void largely affect the magnetic properties; accordingly, the particle size of the magnetic powder and the mixing ratio of the binder to the magnetic powder are important factors in improvement in the magnetic properties thereof.
On the other hand, since the bonded magnet is a mixture of a binder and magnetic material powder, it becomes much more difficult to decompose and recycle the used bonded magnets or bonded magnets rejected as defectives. In particular, in cases where the binder is a thermosetting resin, the bonded magnets are allowed to have a three-dimensional network structure by the setting reaction and generally to become insoluble and non-fusible solids; accordingly, these resin-set bonded magnets have been hard to decompose, and therefore, considered to be unfit for recovering and reusing.
Considering that the waste disposal problem is becoming more serious and the effective use of the resources is important, however, the development of volume reduction technology and of treatment technology for reusing is an urgent problem for the bonded magnets including those having been used as well as those having been rejected as defectives during the processes. In particular, the bonded magnets are composed of metals, metal oxides, etc., and metals are valuables which are more costly than resins which are used as a binder; accordingly, the fact that the possibility of recovering and reusing such costly valuables is limited becomes a bigger issue. In cases where the bonded magnets are composed of rare earth metals such as cobalt and neodymium, the issue becomes much bigger.
Thus, several investigations have been carried out of recycling the bonded magnets; however, for the bonded magnets using thermosetting resins such as epoxy resin as a binder, it is completely impossible to separate and decompose them. Therefore, the only approach taken for resources recovery has been to crush the bonded magnets, mix a small amount of crushed magnets with a virgin magnet powder, add a binder to the mixture, and subjecting the mixture to molding forming.
In the conventional method as above in which the bonded magnets are crushed, the magnet powder and the binder are not satisfactorily separated; therefore, if the mixing ratio of the recycled material to the virgin magnet powder is increased, the density of the bonded magnet, that is, the magnet powder content is decreased, which means that the bonded magnet having the same density as the conventional one cannot be obtained. This means the deterioration of magnetic properties.
In addition, the properties of the magnet powder tend to deteriorate when further pulverizing the raw material. In accordance with the conventional crushing method, since the pulverization of the magnet powder itself is inevitable, the magnetic properties of the bonded permanents magnets produced deteriorate.
Further, due to the pressure applied during the molding forming of the bonded magnets, the particle diameter of the magnet powder becomes smaller. In the conventional method, however, the binder and the magnet powder cannot be separated from each other, and the magnet powder is mixed and reused with its particle size remaining small, which also results in the deterioration of magnetic properties.
After all, the present situation is that the reclamation and resource recovery of the bonded magnets cannot be fully achieved by the conventional method.
Accordingly, in light of the problems described above, the object of the present invention is to provide a method of recycling bonded magnets having the same properties as the currently used one by separating magnetic materials from any type of bonded magnets.
One aspect of the present invention of the present invention is a recycling method of producing magnetic material powder from bonded magnets which are produced by mixing magnetic material powder, as raw material powder, with a binder and subjecting a mixture to molding forming, comprising at least the steps of:
(a) separating and collecting the magnetic material powder from the bonded magnets by removing all or a prescribed percentage of the binder contained in the magnets;
(b) removing all or a prescribed percentage of the particles of diameter smaller than a prescribed particle diameter from the separated and collected magnetic material powder; and
(c) mixing the magnetic material powder, from which the particles of diameter smaller than the prescribed one are removed, with a virgin magnetic material powder in a prescribed mixing ratio, so as to produce a new raw material powder.
Another aspect of the present invention is a method of recycling bonded magnets produced by mixing magnetic material powder, as raw material powder, with a binder and subjecting a mixture to molding forming, comprising the steps. of:
(a) separating and collecting the magnetic material powder from the bonded magnets by removing all or a prescribed percentage of the binder contained in the magnets;
(b) removing all or a prescribed percentage of the particles of diameter smaller than a prescribed particle diameter from the separated and collected magnetic material powder;
(c) mixing the magnetic material powder, from which the particles of diameter smaller than the prescribed one are removed, with a virgin magnetic material powder in a prescribed mixing ratio, so as to produce a new raw material powder; and
(d) mixing the newly produced raw material powder with a prescribed percentage of a binder and subjecting the mixture to molding forming.
Still another aspect of the present invention is the recycling method, wherein the prescribed percentage of the binder to be removed in the step (a) is determined in terms of the prescribed mixing ratio in the step (c).
Yet another aspect of the present invention is the recycling method, wherein the prescribed particle diameter in the step (b) is determined in terms of the prescribed mixing ratio in the step (c).
A further aspect of the present invention is the recycling method, wherein the prescribed ratio in the step (b) is determined in terms of the prescribed mixing ratio in the step (c).
A still further aspect of the present invention is the recycling method, wherein an average particle diameter of the virgin magnetic material powder in the step (c) is determined in terms of the prescribed mixing ratio in the step (c).
A yet further aspect of the present invention is the recycling, wherein the average particle diameter of the virgin magnetic material powder in the step (c) is larger than that of the magnetic material powder contained in the bonded magnets used in the step (a).
A still yet further aspect of the present invention is the recycling method, wherein the prescribed particle diameter in the step (b) set for xcfx8620 to 100 xcexcm.
An additional aspect of the present invention is the recycling method, wherein the bonded magnets contain a thermoplastic resin as the binder and the step (a) is a step of dissolving and separating the binder with a solvent capable of solving the thermoplastic resin.
A still additional aspect of the present invention is the recycling method, wherein the step (a) comprises a step (e) of bringing the bonded magnets into contact with a decomposition solution containing a solvent capable of decomposing the binder in a decomposition bath and heating the decomposition bath to the temperature of 200xc2x0 C. or higher and lower than the critical temperature of the solvent.
A yet additional aspect of the present invention is the recycling method, wherein the solvent in the step (e) is at least one selected from the group consisting of ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, isoprene glycol, triethylene glycol, tetraethylene glycol, 2-methoxyethanol, 2-ethoxyethanol, 2-dimethoxyethanol, 2-isopropoxyethanol, 2-butoxyethanol, 2-isopentyloxyethanol, 2-hexyloxyethanol, 2-phenoxyethanol, 2-benzyloxyethanol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, triethylene glycol monomethyl ether, tripropylene glycol monomethyl ether, tetralin, biphenyl, naphthalene, 1,4-hydroxynaphthalene, naphthol, 1,4-naphthoquinone, pitch, creosote oil, methyl isobutyl ketone, isophrone, 2-hexanone, 2-heptanone, 4-heptanone, diisobutyl ketone, acetonylacetone, phorone, cyclohexanone, methylcyclohexanone and acetophenone.
A still yet additional aspect on of the present invention is the recycling method, wherein the mixing ratio of the binder in the step (d) is made smaller than that of the binder of the bonded magnets used in the step (a).
The bonded magnets provided for the recycling method of the present invention are magnets molded using plastic, rubber, etc. as a binder. In those bonded magnets, prior to molding forming, the magnetic material powder is mixed with the binder. And the bonded magnets are classified depending on the types of the binder and magnetic material powder used. The bonded magnet using a plastics material as a binder is referred to as plastic bonded magnet, and the plastics materials used include, for example, nylon resin, polyphenylene sulfide and epoxy resin. In cases where thermosetting resins such as epoxy resin are used as binders, they are thermoset after the molding forming process. As a magnetic material powder, mainly used are an oxide magnet and a rare earth permanent magnet. As an oxide magnet used are alnico magnet and ferrite magnet, and as a rare earth permanent magnet used are rare earth cobalt alloys, such as SmCo5, SmT17 alloy, and neodymium base alloy such as Nd2Fe14B.
In the bonded magnets, the density and percentage of void affect largely their magnetic properties. Accordingly, when producing the bonded magnets, it is important to regulate the particle size of the magnetic material powder used and the amount of the binder used. The magnetic material powder of which particle size is too small cannot contribute to the magnetic properties of the bonded magnet molded therefrom.
In the bonded magnets molded from magnetic material powder as a raw material powder after mixing the same with a binder, their density is increased due to the pressure applied during molding forming by the injection molding, compression molding, etc. and their void is filled with the binder; accordingly, their magnetic properties are allowed to be excellent. The magnetic material powder once molded into bonded magnets is, however, crushed by the pressure applied during the molding forming, and the particle size of the magnetic material powder separated and collected from the bonded magnets is generally small compared with that of a virgin magnetic material powder.
If the powder having such a small particle size is mixed with a virgin magnetic material powder so as to provide a raw material powder and again molded into a bonded magnet with a binder mixed therein, the magnetic properties of the bonded magnet thus obtained deteriorate because the smaller particles exist which do not contribute to the improvement in magnetic properties or because the density cannot be regulated optimally, and they fell short of our expectations.
According to the present invention, the method of recycling bonded magnets, which are produced by mixing magnetic material powder, as raw material powder, with a binder and subjecting the mixture to molding forming, includes at least the steps of: (a) separating and collecting the magnetic material powder from the bonded magnets by removing therefrom as much binder as possible; (b) removing from the separated and collected magnetic material powder as many particles of diameter less than a prescribed one as possible; (c) mixing the magnetic material powder obtained in the step (b) with a virgin magnetic material powder so as to provide a new raw material powder; and (d) mixing the raw material powder obtained in the step (c) with an additional amount of binder and subjecting the mixture to molding forming.
In the step (c), as many particles of diameter less than a prescribed one as possible are removed from the magnetic material powder separated and collected in the step (b) before mixing the powder with a virgin magnetic material powder. Thus, the particles which have been made smaller by the crushing action of the pressure applied during the molding forming, and therefore, do not contribute to the magnetic properties of the bonded magnets can be removed, in addition, the density of the bonded magnets can be regulated when mixing the magnetic material powder obtained in the step (b) with a virgin magnetic material powder.
Accordingly, the magnetic properties of the bonded magnets are improved and the recycled bonded magnets can be obtained which have the properties equivalent to those of the bonded magnets produced by mixing a virgin magnetic material powder with a binder and subjecting the mixture to molding forming.
The average particle diameter of the virgin magnetic material powder used in the step (c) is larger than that of the magnetic material powder contained in the bonded magnets used in the step (a). In addition, according to the present invention, in the step (c), the average particle diameter of the virgin magnetic material powder, which is mixed with the separated and collected magnetic material powder, is allowed to be larger than that of the magnetic material powder contained in the bonded magnets used in the step (a).
For the magnet powder separated and collected from the bonded magnets once produced through the molding forming process, its particle size is generally smaller than that of the raw material magnetic powder in the first stage due to the crushing action of the pressure applied during the molding forming. The particle diameter distribution, however, can be much more optimized by classifying the particles of the magnet powder and mixing the powder of the classified particles with a virgin magnetic material powder of particle diameter larger than that of the magnet powder. As a result, the magnetic properties of the bonded magnets are improved, and in spite of the use of the separated and collected magnetic material powder, the bonded magnets obtained are allowed to have the properties equivalent to those of the bonded magnets produced by mixing a purely virgin magnetic material powder, as raw material powder, with a binder and subjecting the mixture to molding forming.
For the bonded magnets produced using a thermoplastic resin as a binder, the separating operation in the step (a) can be carried out by the dissolution separation with a solvent capable of dissolving the thermoplastic resin.
As a binder of thermoplastic resin, used are, for example, nylon resin and polyphenylene sulfide. The solvents capable of dissolving thermoplastic resins and being provided for the dissolution separation of the step (a) include, for example, solvents such as acetone, acetylacetone, acetaldehyde, ethyl acetoacetate, methyl acetoacetate, methyl ethyl ketone, diethyl ketone, methyl isobutyl ketone, methyl isopropyl ketone, dimethyl sulfoxide, dimethylformamide, ethyl acetate, isopropyl acetate, butyl acetate, tetrahydrofuran, dioxane, diethyl ether, water, ethanol, methanol, toluene, chloroform, dichloromethane, cresol, phenol, resorcinol, formic acid, glutaric acid, sulfuric acid, phosphoric acid and nitroalcohol; alcohol halide, alcohol solution of alkali metal halide alcohol solution of alkaline earth metal halide, alcohol solution of rhodanic acid, 70% chloral hydrate, monohydroxy cyanide, ethylene glycol and benzyl alcohol.
The solvent used may be a mixed solvent consisting of a plurality of solvents. And heat may be applied so as to dissolve the thermoplastic resins. For polyphenylene sulfide, its solubility in solvent is low at room temperature, therefore, preferably it is dissolved by applying heat about 200xc2x0 C. or higher.
The step (a) preferably includes a step of: bringing the bonded magnets into contact with a decomposition solution containing a solvent capable of decomposing the matter having been set by thermosetting resins in a decomposition bath while heating the bonded magnets at a temperature 200xc2x0 C. or higher and lower than the critical temperature of the solvent.
In this case, as the solvent used is at least one selected from the group consisting of ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, isoprene glycol, triethylene glycol, 2-methoxyethanol, 2-ethoxyethanol, 2-(methoxy methoxy)ethanol, 2-isopropoxyethanol, 2-butoxyethanol, 2-(isopentyloxy)ethanol, 2-(hexyloxy)ethanol, 2-phenoxyethanol, 2-(benzyloxy)ethanol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, triethylene glycol monomethyl ether, tripropylene glycol monomethyl ether, tetraethylene glycol, tetralin, biphenyl, naphthalene, methylnaphthalene, 1,4-hydroxynaphthalene, naphthol, 1,4-naphthoquinone, pitch, creosote oil, methyl isobutyl ketone, isophrone, 2-hexanone, 2-heptanone, 4-heptanone, diisobutyl ketone, acetonylacetone, phorone, cyclohexanone, methylcyclohexanone and acetophenone.
The binder is subjected to chemical decomposition by immersing and heating the bonded magnets in the decomposition solution containing at least one solvent selected from the group consisting of ethylene giycol, propylene glycol, diethylene glycol, dipropylene glycol, isoprene glycol, triethylene glycol, 2-methoxyethanol, 2-ethoxyethanol, 2-(methoxy methoxy)ethanol, 2-isopropoxyethanol,. 2-butoxyethanol, 2-(isopentyloxy)ethanol, 2-(hexyloxy)ethanol, 2-phenoxyethanol, 2-(benzyloxy)ethanol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, triethylene glycol monomethyl ether, tripropylene glycol monomethyl ether and tetraethylene glycol.
Even if the binder is a thermosetting resin such as epoxy resin, the three-dimensional cross-linked chains are subjected to chemical decomposition.
The solvents such as tetralin, biphenyl, naphthalene, methylnaphthalene, 1,4-hydroxynaphthalene, naphthol, 1,4-naphthoquinone, pitch, creosote oil, methyl isobutyl ketone, isophrone, 2-hexanone, 2-heptanone, 4-heptanone, diisobutyl ketone, acetonylacetone, phorone, cyclohexanone, methylcyclohexanone and acetophenone become a liquid phase satisfactory for the decomposition of binders when applying heat, and the liquid phase thermal decomposition reaction of a binder progresses effectively. In this case, even if the binders are the thermosetting resins such as epoxy resin, which are hard to decompose by the conventional methods, they can be decomposed effectively.
Once the binder is decomposed, its function of bonding the bonded magnet having been subjected to setting deteriorates, which results in the disintegration of the bonded magnet. In other words, the bonded magnet becomes unable to hold the magnet powder having been restrained by the binder. Thus the binder components and the magnet powder become easy to separate from each other.
The step (a) allows the binder, including an epoxy resin which is a thermosetting resin having been hard to decompose, to be easily decomposed. And after the decomposition, the magnet powder can be separated and collected from the bonded magnet.
In the steps described above, in order to obtain a high decomposition reaction rate, the temperature is preferably high at which the bonded magnets are brought into contact with a decomposition solution. The reaction rate is acceleratedc sharply, in particular, at 250xc2x0 C. or higher (refer to, for example, Japanese Patent Laid-Open No. 12-198878). However, raising the temperature too high gives rise to problems that: a highly pressure-resistant reactor is required because the pressure becomes too high; the magnet powder becomes hard to collect because the gaseous product caused by the decomposition is increased; the decomposition solution itself may be decomposed; and deterioration reactions such as oxidation of magnet are activated. Accordingly, the temperature at which the bonded magnets are immersed in the decomposition solution is preferably lower than the critical temperature of the solvent contained in the decomposition solution.
For example, the critical temperature of propylene glycol is 351xc2x0 C. As described above, preferably the temperature at which the bonded magnets are brought into contact with the decomposition solution is 200xc2x0 C. or higher and lower than the critical temperature of the solvent used. Meanwhile the inventors found that the reaction rate can be accelerate at even 200xc2x0 C.
In the step (d), the mixing ratio of the binder to the mixed raw material powder is made smaller than that of the binder to the bonded magnet in the step (a) The reason is as follows.
The binder functions to bond the magnet powder; however, the use of too small an amount of binder does not permit the void among the powder particles to be filled in, on the other hand, the use of too large an amount of binder only results in decrease in the density of the bonded magnet. Either case provides optimal magnetic properties.
In the step (a), although it is best to separate and collect 100% of the magnetic material powder from the bonded magnets, the magnetic material powder can sometimes be separated in such a state that part of it holds a slight amount of binder. In that case, in the step (d), the magnetic material powder is mixed with an additional amount of binder and subjected to molding forming in such a state that part of it holds a slight amount of binder. In such a situation, since the binder more than the optimal amount is mixed with the magnetic material powder, the bonded magnets newly produced cannot exhibit optimal magnetic properties.
Therefore, The mixing ratio of the binder to the raw material powder containing the separated and collected magnetic material powder is made smaller than that of the binder to the bonded magnets used in the step (a). This allows the density of the magnetic material to be larger and the magnetic properties to be improved.